Patent GB-763939-A (GB)
Title: Improvements in or relating to shaped polyamide articles and method of forming same
IPC: C08J11/08
Filed: 1952-03-11
Abstract: Articles are formed from particles of polyamides or polyesteramides by sintering under substantially non-oxidizing conditions at a temperature below that at which any substantial amount of molten phase is produced.  The particles have an average diameter of less than 40 microns and are formed under substantially non-oxidizing conditions by precipitation by cooling a solution in a liquid which is a solvent for the polyamide at elevated temperature and a non-solvent at room temperature followed by washing and drying.  A suitable liquid is ethylene glycol or aqueous ethanol.  The powder may also be produced by hammering nylon which has been embrittled with liquid nitrogen. To make articles it is compressed, e.g. at 10-50 tons per square inch, with or without a filler, and the article is then heated to sinter the particles without fusion.  Pressing preferably takes place at room temperature.  The product may be annealed.  The filler may constitute 85 per cent by volume of the total mixture. Suitable fillers which are preferably finely-divided, are diamond dust, silica, carborundum, chalk, alumdum, tungsten carbide, copper, lead, iron, iron oxide, carbon, graphite, molybdenum disulphide, titanium dioxide, various titanates, nylon, of higher melting-point or polytetrafluoroethylene.  To achieve surface porosity a soluble material may be mixed with the nylon powder and leached from the finished product.  It is found that the strength of the finished product increases sharply when the sintering temperature is raised above a certain point (easily ascertained for any polyamide) and sintering should be conducted at a temperature above this but below the melting-point.  Sintering may take place in vacuo or in hot oil.  Preferably the moisture content of the article to be sintered should be low and either the article or the powder may be vacuum dried.  The nylon may be waste, virgin or a mixture.  Examples describe the manufacture of bars or bushings from waste or virgin nylon by the method of the invention and describe physical tests on the products.  In some cases the nylon powder was mixed with graphite or copper powder. Lubricants, e.g. zinc stearate, or hydrogenated cottonseed oil may be mixed with the nylon powder or coated on the die surfaces to improve uniformity of the shaped pieces and facilitate their removal from the mould.  The process is particularly useful for bushings and may also be employed for making thread guides, cams, gears, thrust washers or materials responsive to magnetic or electrical forces or with abrasive properties.  Using a pigment or graphite as the filler, crayons or pencil leads of great strength may be produced.  Specifications 461,236, 461,237, [Group IV], and 763,940 are referred to.ALSO:Articles are formed from particles of polyamides or polyesteramides by sintering under substantially non-oxidising conditions at a temperature below that at which any substantial amount of molten phase is produced.  The particles have an average diameter of less than 40 microns and are formed under substantially non-oxidising conditions by hammering nylon which has been embrittled with liquid nitrogen or, preferably, by precipitation by cooling a solution in a liquid which is a solvent for the polyamide at elevated temperature and a non-solvent at room temperature.  A suitable liquid is ethylene glycol or aqueous ethanol.  The powder, after washing and drying, is compressed e.g. at 10-50 tons per square inch with or without a filler to produce an article which can be handled and which is then heated to sinter the particles without fusion.  Pressing preferably takes place at room temperature.  The product may be annealed.  The filler may constitute 85 per cent by volume of the total mixture.  Suitable fillers which are preferably finely divided are diamond dust, silica, carborundum, chalk, alundum, tungsten carbide, copper, lead, iron, iron oxide, carbon, graphite, molybdenum disulphide, titanium dioxide, various titanates, nylon or polytetrafluoroethylene.  To achieve surface porosity a soluble material may be mixed with the nylon powder and leached from the finished product.  It is found that the strength of the finished product increases sharply when the sintering temperature is raised above a certain point (easily ascertained for any polyamide) and sintering should be conducted at a temperature above this but below the melting point.  Sintering may take place in vacuo or in hot oil.  Preferably the moisture content of the article to be sintered should be low and either the article or the powder may be vacuum dried.  The nylon may be waste, virgin or a mixture. Examples describe the manufacture of bars or bushings from waste or virgin nylon by the method of the invention and describe physical tests on the products.  In some cases the nylon powder is mixed with graphite or copper powder.  Lubricants e.g. zinc stearate, or hydrogenated cotton seed oil may be mixed with the nylon powder or coated on the die surfaces to improve uniformity of the shaped pieces and facilitate their removal from the mould. The process is particularly useful for bushings and may also be employed for making thread guides, cams, gears, thrust washers or materials responsive to magnetic or electrical forces or with abrasive properties.  Using a pigment or graphite as the filler, crayons or pencil leads of great strength may be produced.